Patent Documents 1 and 2 disclose a rankine cycle system that utilizes a refrigerant for cooling an internal combustion engine. For example, such a rankine cycle system includes: the internal combustion engine cooled by a refrigerant circulating therethrough; a gas-liquid separator; a first pump that supplies a refrigerant from the internal combustion engine to the gas-liquid separator, and supplies a liquid-phase refrigerant from the gas-liquid separator to the internal combustion engine again; a steam generator that transfers waste heat from the internal combustion engine to a liquid-phase refrigerant supplied from the gas-liquid separator, and supplies a refrigerant to the gas-liquid separator; a superheater that transfers waste heat from the internal combustion engine to a gas-phase refrigerant supplied from the gas-liquid separator; an expander driven by a refrigerant supplied from the superheater; a condenser that condenses a refrigerant supplied from the expander; and a second pump that supplies a refrigerant from the condenser to the gas-liquid separator.